1. Field of the Invention
The present invention relates to a method of determining an electrical angle in a synchronous motor and an apparatus for the same.
2. Description of the Related Art
As an apparatus for measuring an electrical angle with high precision in a synchronous motor even in the cased state of a rotor or at a low-revolving speed of the rotor, the applicant of the present invention has proposed an apparatus for observing a behavior of electric current flowing through each phase and determining an electrical angle based on the observed behavior (JAPANESE PATENT LAYING-OPEN GAZETTE No. 7-177788). In the case of a three-phase synchronous motor, for example, the proposed apparatus observes behaviors of electric currents flowing through any two phases, thereby unequivocally determining the electrical angle.
The proposed apparatus can determine the electrical angle with high precision even when the rotor is at a stop or at a low revolving speed. Foreign noises, however, sometimes occur in the process of determining the electrical angle in a synchronous motor with the proposed apparatus. The following describes this drawback in detail. The proposed apparatus carries out measurement while successively varying the combination of phases to which the voltage is applied. It is assumed that a three-phase synchronous motor is driven by making electric currents Iu, Iv, and Iw flow through U, V, and W phases. As shown in FIGS. 18A and 18B, the proposed apparatus returns the driving currents to zero at a predetermined timing, applies a voltage between U-WV phases to measure an electric current flowing through the U-phase at a timing m1 in FIG. 18B, applies the voltage between V-WU phases to measure an electric current flowing through the V-phase at a timing m2, and applies the voltage between W-UV phases to measure an electric current flowing through the W-phase at a timing m3. Since the sum of the electric currents flowing through the three phases is equal to zero, electric current sensors are required at two difference places, for example, in the U-phase and the V-phase.
In case that the respective phase currents are measured three times while a voltage is applied between each combination of phase-coils, several milliseconds are required for determination of the electrical angle. In order to ensure a certain level of precision, measurement of the electrical angle is required at every ten-odd milliseconds. The electric currents for measurement are thus flown for a period of several milliseconds at the intervals of ten-odd milliseconds as shown in FIGS. 18A and 18B. The known procedure returns the driving voltage applied to the synchronous motor to zero before application of a voltage for measurement, and resumes application of the driving voltage of the original level after conclusion of the measurement. The foreign noises occur in this process. The foreign noises become larger with an increase in torque current to the synchronous motor.